The present invention relates to a reactor core for a boiling water nuclear reactor with a plurality of vertical fuel assemblies, each fuel assembly containing a plurality of vertical fuel rods with enriched nuclear fuel material, which fuel rods are arranged between a bottom tie plate and a top tie plate in a surrounding vertical fuel channel. Each fuel assembly is formed with an inlet for water for conducting water in through the bottom tie plate, through the space between the fuel rods in the vertical fuel channel, and out through the top tie plate. Further, each fuel assembly is arranged with intermediate gaps with respect to adjoining fuel assemblies and possibly with a channel, internally arranged in the fuel assembly, for conducting water through the gaps and through the internal channel (if any) in the vertical direction from below and upwards through the core.
Usually, each fuel rod in a fuel assembly of the kind described above is arranged with the same enrichment content of fissile material (hereinafter referred to as enrichment only) in its entire length.
During operation of a bailing water nuclear reactor with a core containing several fuel assemblies of the above-mentioned kind, there may be a risk of so-called dryout, that is, that the water film present on the surface of each fuel rod disappears or is broken down in restricted areas, which locally leads to a considerably deteriorated heat transfer between the fuel rod and the water conducted through the fuel assembly, resulting in a greatly increased wall temperature of the fuel rod. The increased wall temperature may result in damage with serious consequences arising on the fuel rod.
In current designs with the same enrichment in the whole length of each fuel rod, in order to achieve a sufficient dryout margin, the requirements for the effect which is otherwise attainable are lowered. The present invention makes it possible to improve the dryout margin without deteriorating the attainable effect. The favourable result is achieved according to the invention by a redistribution of the fuel while achieving a special enrichment distribution in the longitudinal direction of the fuel rods.
What more particularly characterizes the present invention is that each one of at least the main part of the fuel rods, central rods, which in a fuel assembly are surrounded by fuel rods, edge rods, which are located close to a water gap or an internal water channel, is arranged with a ratio of the enrichment in the central rod in question to the medium enrichment for the central rods and the edge rods in a horizontal section, which is lower for an upper part than for a lower part of the central rod.
Preferably, each one of at least the main part of the edge rods is arranged with a ratio of the enrichment in the edge rod in question to the medium enrichment for the central rods and the edge rods in a horizontal section, which is higher for the upper part than for the lower part of the edge rod.
The length of the upper part suitably constitutes one-fourth to two-thirds, preferably one-third to two-thirds, of the active length of tile fuel rod and the lower part the remainder of the active length of the fuel rod. By the active length of the fuel rod is meant that axial part of the fuel rod which contains nuclear fuel material.
According to an advantageous embodiment of the invention, which provides a good shutdown margin for the reactor, the medium enrichment of the central rods and the edge rods in a horizontal section is 5-15% lower in the area of the upper part of the fuel rods than in tile area of the lower part of the fuel rods.
It is known per se to arrange fuel rods with different enrichments in different parts of the length of the fuel rods in order to influence, in other ways than those stated above, the properties of the reactor core and the operation of a nuclear reactor.